Hot melt adhesives (HMAs) are thermoplastic solid materials at room temperature and generally applied in molten or liquid state. Hot melt pressure sensitive adhesives (HMPSAs) are the class of HMAs which remain tacky after solidification and have the ability to adhere on surfaces upon application of pressure.
HMAs are packed in different forms in respect to the characteristics of the product and end user requirements. HMA and specifically HMPSA present a variety of handling difficulties and contamination risks due to the tacky nature of the products unless protected with a tack-free (i.e. “non-tacky”) film or coating. Moreover, high tack HMPSA formulations which are required in certain applications easily deform and cold flow unless supported during shipment or when stored for prolonged times at elevated temperatures. As a consequence, tack-free HMPSA packaging techniques are developed and tried to be improved in various ways.
Many examples in the state of the art mention casting HMA and specifically HMPSA in molten state inside silicone coated containers such as carton boxes, drums, plastics, papers or disposable trays in different sizes. The material is subsequently cooled at ambient conditions or in large refrigerators during a suitable period of time, stacked, packed and prepared for dispatch. However, there are major disadvantages present as the product is cooled and solidified during a long period of time before delivery; additional shelves are required for cooling process; hazard potential is high as hot liquid is handled; the cost of disposable containers are costly for the manufacturer; the packaging needs to be separated with a significant physical force by the user before adding the material in the melt pot and the packaging is disposed at the end of the processing.
In U.S. Pat. No. 5,373,682 a process is described wherein a molten HMPSA is directly poured or pumped into a cylindrical plastic tube which is in contact with the heat sink to remove the excess heat from the film. The molten adhesive is fused to the surrounding film and a non-blocking adhesive package is provided which melts simultaneously with the adhesive and does not cause significant build up of packaging in the melt pot. In U.S. Pat. No. 5,865,927 a HMPSA with tack-free surface is described wherein the adhesive is extruded through a die orifice; the surface of the adhesive is sprayed with a molten film forming polymeric material, while the adhesive is still in molten state. The surface of the coated adhesive is subsequently heat treated in order to provide a continuous film and the adhesive mass is cooled to a temperature suitable for handling.
In addition to casting, pouring molten HMA in mold cavities or plastic tubes and coextrusion of adhesive together with a non-tacky coating, there are also examples available in the prior art for pelletizing or squeeze-cutting (pillows) of HMPSA and powdering with non-tacky components such as organic (polymeric) material (e.g. phenolic resins)-containing powders, individually coating pelletized materials with non-tacky materials or filling the adhesive into plastic packaging films. EP 0 410 914 discloses a method for preparation of non-tacky, free flowing polymeric particles by extruding the molten plastic material into a cooling fluid bath containing non-sticky material, that is compatible with the plastic material, cutting the plastic material into portions, separating the portions from the cooling fluid and subsequently contacting individual particles with a second non-sticky material preferably with powdered polyolefin waxes. U.S. Pat. No. 6,716,527 discloses a free flowing HMPSA in form of granulates wherein an outer layer of the material is not pressure sensitive up to 45° C. HMPSA in molten form is granulated underwater, dried and individually sprayed with a fine droplet liquid coating component comprised of polyethylene wax, defoamer, film former and water wherein individual pellets are covered at least 90% by the coating component. Powdering or individual coating of the HMPSA granulates or pieces have major drawbacks since the surface to volume ratio of the coating is significantly high and a high risk exists that the product is diluted and highly contaminated with the coating material.
EP 0 469 564 discloses a method wherein HMA is firstly formed as uniform separate portions, solidified and finally enveloped in larger portions by a plastic packaging film. The plastic packaging material is a component of the adhesive or chemically compatible with the adhesive hence the packaging can be melted with the adhesive in the pot. However, both the adhesive and the film come into contact in cold state and there is no fuse or substantial interactions between the film and the adhesive. Hence, during the melting period, adhesive and film melts separately and due to the air pockets between the film and the portions of the HMA, the film is pushed and gathered on the top of the melt pot. Consequently, the film cannot be melted properly and floats on the surface of the tank, so-called “jelly fish effect” in the prior art, and the film subsequently carbonizes and blocks the filters in the melt pot.